This invention relates to improvements in or relating to ceramic piezoelectric ink jet print heads of the kind having an ink channel for connection to an ink ejection nozzle and to a reservoir for the ink, and a piezoelectric wall actuator which forms part of the channel and is displaceable in response to a voltage pulse thereby generating a pulse in liquid ink in the channel due to a change of pressure therein which causes ejection of a liquid droplet from the channel. Such print heads are referred to hereafter as piezoelectric ceramic ink jet print heads.
Examples of such print heads as described, for example, in EP-A-277703, EP-A-278590 and EP-A-364136 are shown in FIGS. 1-3. FIGS. 1 and 2 are different sectional views of the same ink jet printhead, and FIG. 3 is a view similar to that of FIG. 1 showing another form of printhead.
An enlarged view of a channel construction for which the teachings of the invention are particularly well suited is shown in FIG. 4. A portion of the channel of FIG. 4 is illustrated in FIGS. 5-8 including various examples of multi-layer coatings after being deposited according to the teachings of the present invention.
One form of ink jet printhead 10 comprises a multiplicity of parallel ink channels 12 forming an array in which the channels are mutually spaced in an array direction perpendicular to the length of the channels. The channels are formed at a density of two or more channels per mm. in a sheet 14 of piezoelectric material, suitably PZT, poled in the direction of arrows 15 and are defined each by side walls 16 and a bottom surface 18, the thickness of the PZT being greater than the channel depth. The side walls 16 are generally at an angle of no more than 10xc2x0 from the normal to the bottom wall. The channels 12 are open topped and in the printhead are closed by a top sheet 20 of insulating material which is thermally matched to the sheet 14 and is disposed parallel to the surfaces 18 and bonded by a bonding layer 21 to the tops 22 of the walls 16. The channels 12 on their side wall surfaces are lined with a metallised electrode layer 34. It will be apparent therefore that when a potential difference of similar magnitude but opposite sign is applied to the electrodes on opposite faces of each of two adjacent walls 16, the walls will be subject to electric fields in opposite senses normal to the poling direction 15. The walls are in consequence deflected in shear mode.
Referring now to FIG. 2, the channels 12 therein are provided on facing walls 16 thereof with metallised electrodes 34 which extend from the edges of the tops 16 of the walls down the walls to a location well short of the bottom surface 18 of the channels. There is an optimum metallisation depth which gives maximum wall displacement at about the mid-height of the walls depending on the distribution of wall rigidity. In this form the walls are of the so-called cantilever type.
In FIG. 2, it will be seen that the channels 12 comprise a forward part 36 of uniform depth which is closed at its forward end by a nozzle plate 38 having formed therein a nozzle 40 from which droplets of ink in the channel are expelled by activation of the facing actuator walls 16 of the channel. The channel 12 rearwardly of the forward part 36 also has a part 42 of lesser depth extending from the tops 22 of the walls 16 than the forward part 36. The metallised plating 34 which is on opposed surfaces of the walls 16 occupies a depth approximately one half that of the channel side walls but greater than the depth of the channel part 42 so that when plating takes place the side walls 16 and bottom surface 18 of the channel part 42 are fully covered whilst the side walls in the forward part 36 of the channel are covered to approximately one half the channel depth in that part. One suitable electrode metal used is an alloy of nickel and chromium, i.e. nichrome. Alternatively, aluminium provides a high conductivity electrode and the metal track in the part 42 is suitable for applying a wire bond connection. Aluminium in particular requires to be coated with a layer of passivation to inhibit electrolysis and bubble formation or corrosion which could occur if the electrode is in direct contact with the ink.
It will be noted that a droplet liquid manifold 46 is formed in the top sheet 20 transversely to the parallel channels 12 which communicates with each of the channels 12 and with a duct 48 which leads to a droplet liquid supply (not shown).
In the arrangement shown in FIG. 3, wherein elements common with the embodiment of FIGS. 1 and 2 are identified by the same reference numerals as in FIGS. 1 and 2, a sheet 14 is employed therein having upper and lower regions poled in opposite senses as indicated by the arrows 15. A sheet 50xe2x80x2 of glass or other insulating material is employed as a stiffening means for the sheet 14 of piezo-electric material. The electrodes 34 are deposited so as to cover the facing channel side walls from the tops thereof down to a short distance from the bottoms of the channels so that a region of each side wall extending from the top of the channel and poled in one sense and a substantial part of a lower region of the side wall poled in the reverse sense are covered by the relevant electrode. Thus, it will be appreciated that the arrangement described operates to deflect the channel side walls into chevron form. Other forms of ink jet printhead having an array of ink channels separated by piezoelectric wall actuators described in the art are also suitable for the application of the process of this invention.
The invention is concerned with passivation of the walls of the channels; that is, the deposition of a protective layer on the walls by coating. The purpose of the passivation is to provide a coating acting as an electron or ion or ink barrier and therefore to protect the channel walls from attack by the ink and/or to protect the ink from the channel walls. Protection of the channel walls from the ink is particularly desirable where the ink is aqueous or otherwise electrically conductive.
Wherexe2x80x94as is the case in the example given abovexe2x80x94the channel includes opposed walls comprising piezoelectric ceramic material and is provided with electrodes for connection to voltage pulse generating means, passivation is particularly desirable to protect the electrodes from the ink and also to insulate the ink from the electrodes, and more particularly the fields generated by the electrodes, especially where the ink is a dispersion. In one embodiment of this form of ink jet print head, the channels are formed with opposed side walls and a bottom wall all of piezoelectric ceramic material, e.g. by cutting or machining an open channel from a block of the material, and a top wall which closes the channel. In this embodiment, in general the side walls and bottom wall are passivated.
IBM Technical Disclosure Bulletin, Vol. 23, No. 6, November 1980, page 2520 discloses a method for passivation of an ink jet silicon nozzle plate whereby a first overcoat of thermal SiO2 is applied to a silicon substrate followed by a second overcoat of glow discharge silicon carbon. Formation of the first overcoat generally entails substrate temperatures of the order of 900xc2x0 C.
EP-A-0 221 724 discloses an ink jet printer nozzle having a substrate of silicon or glass and a coating resistant to corrosion by aqueous and non-aqueous inks. The coating comprises respective layers of silicon nitride, silicon nitride with aluminium nitride, and aluminium nitride. Sputtering, Chemical Vapour Deposition (CVD) and evaporation are given as suitable techniques for forming the coating. Typical substrate temperatures are given as 700-800xc2x0 C. and, as described, ion-assisted deposition is a line-of-sight coating process.
U.S. Pat. No. 4,678,680 discloses the use of an ion beam implanting device to implant ions in the aperture plate of an ink jet printer of the continuous stream type, thereby improving the corrosion resistance of the aperture plate.
IBM Technical Disclosure Bulletin, Vol. 22, No. 8, January 1979, page 3117 discloses a method of depositing a coating material such as titanium on to the bore of a nozzle using ion plating. This method relies on resputtering of that coating material initially deposited near the mouth of the bore of the nozzle so as to achieve coating further inside the bore.
Achieving the substantially continuous coating of the channel walls of a ceramic piezoelectric printhead that is required for effective passivation gives rise to particular problems, however. One problem is that certain areas of especially the lower parts of the side walls of a channel cannot readily be coated by procedures which require line-of-sight between the coating source and the surface to be coated because, when the source is appropriately located relative to the channel for deposition of a layer on a side wall, these lower parts will be in the shadow of the upper part of the opposite wall. Moreover, this problem increases with the depth of the channel relative to its width (referred to hereafter as the xe2x80x9caspect ratioxe2x80x9d of the channel).
Another problem particular to this type of ink jet printhead is caused by the granular structure of the piezoelectric material from which the printhead is made: grain-cluster pull-out occurs to a greater or lesser extent during formation of the channel, leaving walls having microscopic crevices, undercuts and overhangs.
These problems may be understood more clearly by reference to FIG. 4 which is a very much enlarged view of a channel 112 defined by walls 116 and 116a. The coating of the surface 150 of the wall 116 using conventional line-of-sight deposition procedures such as ion implantation or ion plating, which require line of sight 152 between the coating source and the surface to be coated, is not possible. It is likewise impossible to coat undercut zones such as 154, 156 and 158 even though they are not shadowed by the opposite wall 116a of the channel. Consideration of the geometry will also show that this second problem becomes even more acute with increase in the aspect ratio of the channel; that is, the ratio of the depth of the channel to its width: the greater the aspect ratio, the more acute is the maximum possible angle between (a) the line of sight be between the source and the channel bottom and (b) the plane of the channel wall, and thus the greater the area of undercut that is placed in shadow by the overhang above. Such acute angles also present problems in achieving a uniform and continuous coating over the walls and channel bottom since the capture efficiency of coating materials varies with the angle of incidence.
Yet another problem is caused by the manner of construction of ceramic piezoelectric ink jet printheads of the type mentioned at the beginning of the description: the configuration of the electrodes, which are arranged so as to generate an electric field perpendicular to the direction of polarisation of piezoelectric material thereby to displace the piezoelectric wall actuators in shear mode makes it very difficult if not impossible to perform any subsequent repoling of the piezoelectric material once passivation has taken place. Furthermore, ink jet print heads of the type in question are preferably made from a high activity piezoelectric ceramic having a Curie temperature (i.e. the temperature Tc at which the material is no longer capable of retaining polarisation) of the order of 150xc2x0 C. to 250xc2x0 C. The coating process should be performed at a lower temperature, suitably 50xc2x0 C. to 100xc2x0 C. below the Curie temperature, to avoid accelerated aging or depoling of the piezoelectric material. The use of conventional chemical vapour deposition or plasma-enhanced chemical vapour deposition coating procedures, which generally employ temperatures substantially in excess of 200xc2x0 C., e.g. 300xc2x0 C. or 500xc2x0 C. or even more, therefore necessitates repolarisation following passivation if printhead activity (and hence efficiency) is not to be lost. To avoid repolarisation following passivation, a coating process temperature of less than 200xc2x0 C., and preferably not more than 100xc2x0 C., is required, the lower temperatures permitting the use of more active materials.
At lower temperatures, either coating is not achievable at all with these procedures or it is only achievable at an unacceptably slow rate and in any event there is another problem which is that coating thicknesses tend to decrease from top to bottom of the channel and thus under the conditions required to achieve the desired thickness of coating towards the bottom of the channels, the deposition of coatings of excessive thickness at the top is unavoidable, and as the tendency for the coatings to have defects such as unrelieved internal stresses increases with thickness, the risk of obtaining a defective coating is increased. This problem becomes particularly acute where the channel has an aspect ratio of more than 2:1, e.g. 3:1 or more. For example, for some channels where the aspect ratio is 4:1 or more, coating thicknesses of as much as one half or one micron may be found in the upper parts of the channel under the conditions required to achieve a desired coating thickness of 50-100 nm lower down. Channels having an aspect ratio of 3:1 or more are hereafter referred to as deep channels.
The present invention aims to solve the above problems.
According to the present invention, there is provided a process for the passivation of the channel walls of a deep channel ink jet print head channel of ceramic piezoelectric material by the deposition of a coating comprising inorganic material, the process comprising:
(a) providing an ink jet print head component containing said channel and
(b) while maintaining the bulk temperature of the actuating component which contains said channel at a temperature of below 200xc2x0 C. and at which not more than 30% depolarisation of the material occurs during passivation, exposing the surface of the channel walls to be passivated to a homogenised vapour of the coating material, said vapour having undergone multiple scattering during transport thereof from the source of the vapour to said surface.
By a homogenised vapour, we mean that the chemical constituents of the vapour used by the process have a substantially uniform distribution, so that the coating deposited approaches and preferably attains chemical homogeneity in the surface layer.